Although this invention can be used in many metal cutting processes, it is particularly useful in gear shaving. In gear shaving, the distance between the centerlines of the cutter and the workpiece is reduced by either feeding the workpiece into the cutter or feeding the cutter into the workpiece. This distance is called the center distance.
Before shaving production gears to a specific size, the finished size shaving center distance (Csh) must be determined. This value is the smallest distance between the centerlines of the shaving cutter and the workpiece used to produce the workpiece to finished size.
The first step in determining (Csh) is to either calculate or experimentally measure a test center distance (Ct). A theoretical value for (Ct) may be calculated by using gear and tool geometry information. Alternatively, an experienced machine operator can find (Ct) experimentally by bringing the shaving cutter and workpiece into mesh and rotating them while adjusting (Ct) until the correct amount of backlash is observed. Once (Ct) is determined, a test workpiece is shaved and measured for size. If the workpiece is within the acceptable size tolerance, it is kept and (Csh) is set to equal (Ct). Otherwise the workpiece is scrapped, (Ct) is adjusted and another test workpiece is shaved.
Theoretically, it should be possible to use the shaving center distance (Csh) to consistently shave workpieces to the correct size. However, in practice, this is seldom the case. Typically, workpiece size will gradually become smaller as repeated workpieces are shaved. In addition, the first workpiece shaved per shift will vary in size from day to day. These observed changes in workpiece size are the result of thermal instability of the machine. In other words, as the machine heats up or cools down the actual center distance that the workpiece is shaved to varies because heat causes parts of the machine to expand and contract. This growth and shrinkage if left unchecked is usually significant enough to cause the workpiece to fail the size tolerance test over time. Therefore some type of size compensation is usually administered.
One way to try to avoid workpiece size inconsistency is to control the temperature of the work environment. Controlling the coolant temperature has been employed, but this method does not account for all of the thermal factors.